A local area network is a high speed, multiple access, private communication network. Depending upon the communication medium, for example, cable, wire or optical fiber, the local area network has the capability both to handle data at rates exceeding 10 M bps and to connect more than 50,000 devices.
Local area networks are currently experiencing widespread acceptance as a means for providing interconnection and communications among host computers, terminals and peripheral equipment located throughout a single building or group of buildings. These local area networks have emerged from a variety of different architectures based on the point-to-point, star, ring, bus and tree topologies.
Point-to-point topology causes each device in the network to be connected to every other device. As such, point-to-point topology is limited to small networks because of high associated costs of implementation and difficulty in monitoring network activity due to the lack of a central communications control device.
Star topology is popular because it affords centralized control of the network from a single switching node to which all devices in the local area network are connected. In this topology, device connections are simple and can often be accomplished using existing wire.
Ring topology is one in which each device is connected to exactly two other devices in the local area network. A message token is passed to each device in sequence around the ring in order to permit each device an opportunity to send or receive information. While this topology reduces considerably the number of device connections over the point-to-point and star topologies, it does not offer centralized communications control. Moreover, when a device in the network experiences a failure, the entire local area network may be disabled until the failed device is removed or replaced.
Bus topology is characterized by the plurality of devices in the network being connected individually to a single, bidirectional, broadband communications medium referred to as a bus. Generally, each device has a unique address and can gain bus access by a centralized or distributed interrupt priority scheme. Message collisions are avoided through the use of specific protocols. Tree topology resembles several bus networks linked together via a common bus. This topology is applicable to networks which must operate over long distances.
As stated above, several different media can be used to carry local area network communications. Considerations regarding network topology, maximum distance between nodes, volume of information to be transmitted, and speed of transmission are critical in selecting a particular communications medium. Physical limitations such as plenum, conduit sizes, and routing plans in the building also affect the choice of the medium. Finally, for some network topologies, user accessibility to the bus for passive tapping is extremely important.
Commercially available local area networks such as Ethernet (Ethernet is a trademark of Xerox Corporation) make use of the various topologies and transmission media described above. At the present time, coaxial cable appears to be the medium of choice because it is capable of supporting very high bit rates. However, local area networks relying on a coaxial cable transmission medium require the use of complex repeaters and regenerators to maintain signal integrity and to obtain reasonable transmission distances for the network. Coaxial cable is also quite expensive and bulky which presents problems when wiring a local area network in an existing building. Also, since many present local area networks involve some type of bus structure and because the bus structure is physically long, there are many signal timing and synchronization problems caused by dispersive and reflective effects of the transmission medium which must be overcome by the collision detection circuitry and bus coupler or tap spacing, at the very least. Finally, many of the present local area networks lack modularity and modular interconnectivity.